1. Field of the Invention
The present invention relates to a waterproof nut which may be used, for example, inside a water storage tank, and more particularly to a waterproof nut which may be employed in conditions in which it comes into contact with water or in which a liquid that may produce condensation is present, and which may also be used for a joint or the like in piping. The present invention also pertains to a method of manufacturing a waterproof nut of the type described above.
2. Description of the Related Art
One type of known water storage tank is constituted by panel units which are tightly joined together by means of nuts and bolts which are screwed and tightened thereon. Since the nuts and bolts employed in a high-humidity atmosphere, as in the case of a water storage tank, must be resistant to corrosion, it is general practice to subject such nuts and bolts to plating or chromating, or to make them of a stainless steel material or a synthetic resin material. These anticorrosion treatments, however, cannot prevent corrosion from chlorine gas generated from the water contained in the tank, and even nuts and bolts which are made of a stainless steel material are corroded by this chlorine gas. Nuts and bolts which are made of a synthetic resin material have insufficient mechanical strength for obtaining the desired level of performance, and are therefore unsatisfactory for industrial purposes.
There is a known method in which, after panel units have been fastened together by nuts and bolts, the nut and bolt assemblies are externally covered with caps made of a resin. This method, however, involves troublesome operations, and has the disadvantage that the caps may come off the nut and bolt assemblies during use.
To overcome these disadvantages, the applicant of the present invention has already proposed an improved waterproof nut (see Japanese Utility Model Laid-Open No. 99998/1984). The structure of his waterproof nut is shown in FIG. 10. More specifically, a cap nut body 20 which has a cap-shaped portion 21 is employed. The surface of the cap-shaped portion 21 is machined so that projections and recesses are formed thereon, and is then coated with a synthetic resin layer 22. This proposed prior art has already been widely used as a nut for, in particular, water storage tanks.
This type of nut, however, necessitates a complicated manufacturing process, which leads to an increase in the production cost. More specifically, the formation of the cap nut body 20 requires much more time and labor than ordinary nut bodies, so that the production cost of the former can be at least ten times that of the latter.
The necessity of providing the cap-shaped portion 21 on the nut body 20 will be explained below.
The nut of the above-described type is generally produced by setting the cap nut body 20 in a mold and charging a synthetic resin into the mold. In this case, if an ordinary nut body with no cap-shaped portion 21 is employed, a core which has the same configuration as that of the bolt which is to be screwed into the completed nut is screwed into the nut body, and the nut body is set in the mold in this state. The synthetic resin is then charged, and, after the resin has hardened, the core is unscrewed from the nut body which is still in the mold.
In this process, it is impossible to prevent the charged synthetic resin (liquid) from entering the small space defined between the nut body and the threaded portion of the core, but any resin in that small space makes it extremely difficult to unscrew the core from the nut body. In order to solve this problem, the cap-shaped portion 21 is formed on the nut body 20, and the synthetic resin is thereby prevented from entering the small space.
In addition, since the inner diameter of the synthetic resin portion is inevitably reduced by mold shrinkage, the thread engagement area between the nut and a bolt may become so tight that the nut cannot be smoothly screwed onto the bolt, which means that the working efficiency of the fastening operation is greatly reduced, and it is not possible to obtain the required clamping force.